[PATCH v2 5/8] security/brute: Mitigate a fork brute force attack

[John Wood]

In order to mitigate a fork brute force attack it is necessary to kill
all the offending tasks. This tasks are all the ones that share the
statistical data with the current task (the task that has crashed).

Since the attack detection is done in the task_fatal_signal LSM hook
only is needed to kill the other tasks that share the same statistical
data, not the ones that have the same group_leader that the current task
since the latter are in the path to be killed.

When the SIGKILL signal is sent to the offending tasks, the
brute_kill_offending_tasks function will be called in a recursive way
from the task_fatal_signal LSM hook due to a small crash period. So, to
avoid kill again the same tasks due to a recursive call of this
function, it is necessary to disable the attack detection for this fork
hierarchy.

To disable this attack detection, empty the last crashes timestamps list
and avoid to compute the application crash period if the size of this
list is zero.

Signed-off-by: John Wood <john.wood at gmx.com>
---
 security/brute/brute.c | 144 ++++++++++++++++++++++++++++++++++++++---
 1 file changed, 135 insertions(+), 9 deletions(-)

diff --git a/security/brute/brute.c b/security/brute/brute.c
index 223a18c2084a..a1bdf25ffcf9 100644
--- a/security/brute/brute.c
+++ b/security/brute/brute.c
@@ -3,6 +3,7 @@
 #define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

 #include <asm/current.h>
+#include <asm/rwonce.h>
 #include <linux/bug.h>
 #include <linux/cache.h>
 #include <linux/compiler.h>
@@ -14,9 +15,14 @@
 #include <linux/limits.h>
 #include <linux/list.h>
 #include <linux/lsm_hooks.h>
+#include <linux/pid.h>
 #include <linux/printk.h>
 #include <linux/refcount.h>
+#include <linux/rwlock.h>
 #include <linux/sched.h>
+#include <linux/sched/signal.h>
+#include <linux/sched/task.h>
+#include <linux/signal.h>
 #include <linux/slab.h>
 #include <linux/spinlock.h>
 #include <linux/sysctl.h>
@@ -295,23 +301,39 @@ static void brute_task_execve(struct linux_binprm *bprm)
 }

 /**
- * brute_stats_free() - Deallocate a statistics structure.
- * @stats: Statistics to be freed.
+ * brute_timestamps_free() - Empty a last crashes timestamp list.
+ * @timestamps: Last crashes timestamps list to be emptied.
  *
- * Deallocate all the last crashes timestamps list entries and then the
- * statistics structure. The statistics to be freed cannot be NULL.
+ * Empty the last crashes timestamps list and deallocate all the entries. This
+ * list cannot be NULL.
  *
- * Context: Must be called with stats->lock held and this function releases it.
+ * Context: Must be called with stats->lock held.
  */
-static void brute_stats_free(struct brute_stats *stats)
+static void brute_timestamps_free(struct list_head *timestamps)
 {
 	struct brute_timestamp *timestamp, *next;

-	list_for_each_entry_safe(timestamp, next, &stats->timestamps, node) {
+	if (list_empty(timestamps))
+		return;
+
+	list_for_each_entry_safe(timestamp, next, timestamps, node) {
 		list_del(&timestamp->node);
 		kfree(timestamp);
 	}
+}

+/**
+ * brute_stats_free() - Deallocate a statistics structure.
+ * @stats: Statistics to be freed.
+ *
+ * Deallocate all the last crashes timestamps list entries and then the
+ * statistics structure. The statistics to be freed cannot be NULL.
+ *
+ * Context: Must be called with stats->lock held and this function releases it.
+ */
+static void brute_stats_free(struct brute_stats *stats)
+{
+	brute_timestamps_free(&stats->timestamps);
 	spin_unlock(&stats->lock);
 	kfree(stats);
 }
@@ -426,6 +448,104 @@ static u64 brute_get_crash_period(struct brute_timestamp *new_entry,
 	return jiffies64_to_msecs(jiffies);
 }

+/**
+ * brute_disabled() - Test the fork brute force attack detection disabling.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * The fork brute force attack detection enabling / disabling is based on the
+ * last crashes timestamps list current size. A size of zero indicates that this
+ * feature is disabled. A size greater than zero indicates that this attack
+ * detection is enabled.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * It's mandatory to disable interrupts before acquiring the lock since the
+ * task_free hook can be called from an IRQ context during the execution of the
+ * task_fatal_signal hook.
+ *
+ * Return: True if the fork brute force attack detection is disabled. False
+ *         otherwise.
+ */
+static bool brute_disabled(struct brute_stats *stats)
+{
+	unsigned long flags;
+	bool disabled;
+
+	spin_lock_irqsave(&stats->lock, flags);
+	disabled = !stats->timestamps_size;
+	spin_unlock_irqrestore(&stats->lock, flags);
+
+	return disabled;
+}
+
+/**
+ * brute_disable() - Disable the fork brute force attack detection.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * To disable the fork brute force attack detection it's only necessary to empty
+ * the last crashes timestamps list. So, a list size of zero indicates that this
+ * feature is disabled and a list size greater than zero indicates that this
+ * attack detection is enabled.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * Context: Must be called with stats->lock held.
+ */
+static void brute_disable(struct brute_stats *stats)
+{
+	brute_timestamps_free(&stats->timestamps);
+	stats->timestamps_size = 0;
+}
+
+/**
+ * brute_kill_offending_tasks() - Kill the offending tasks.
+ * @stats: Statistical data shared by all the fork hierarchy processes.
+ *
+ * When a fork brute force attack is detected it is necessary to kill all the
+ * offending tasks involved in the attack. In other words, it is necessary to
+ * kill all the tasks that share the same statistical data but not the ones that
+ * have the same group_leader that the current task since the latter are in the
+ * path to be killed.
+ *
+ * When the SIGKILL signal is sent to the offending tasks, this function will be
+ * called again from the task_fatal_signal hook due to a small crash period. So,
+ * to avoid kill again the same tasks due to a recursive call of this function,
+ * it is necessary to disable the attack detection for this fork hierarchy.
+ *
+ * The statistical data shared by all the fork hierarchy processes cannot be
+ * NULL.
+ *
+ * Context: Must be called with stats->lock held.
+ */
+static void brute_kill_offending_tasks(struct brute_stats *stats)
+{
+	struct task_struct *p;
+	struct brute_stats **p_stats;
+
+	if (refcount_read(&stats->refc) == 1)
+		return;
+
+	brute_disable(stats);
+	read_lock(&tasklist_lock);
+
+	for_each_process(p) {
+		if (p->group_leader == current->group_leader)
+			continue;
+
+		p_stats = brute_stats_ptr(p);
+		if (READ_ONCE(*p_stats) != stats)
+			continue;
+
+		do_send_sig_info(SIGKILL, SEND_SIG_PRIV, p, PIDTYPE_PID);
+		pr_warn_ratelimited("Offending process %d (%s) killed\n",
+				    p->pid, p->comm);
+	}
+
+	read_unlock(&tasklist_lock);
+}
+
 /**
  * brute_task_fatal_signal() - Target for the task_fatal_signal hook.
  * @siginfo: Contains the signal information.
@@ -433,7 +553,8 @@ static u64 brute_get_crash_period(struct brute_timestamp *new_entry,
  * To detect a fork brute force attack is necessary that the list that holds the
  * last crashes timestamps be updated in every fatal crash. Then, an only when
  * this list is large enough, the application crash period can be computed an
- * compared with the defined threshold.
+ * compared with the defined threshold. If at this moment an attack is detected,
+ * all the offending tasks must be killed.
  *
  * It's mandatory to disable interrupts before acquiring the lock since the
  * task_free hook can be called from an IRQ context during the execution of the
@@ -450,6 +571,9 @@ static void brute_task_fatal_signal(const kernel_siginfo_t *siginfo)
 	if (WARN(!*stats, "No statistical data\n"))
 		return;

+	if (brute_disabled(*stats))
+		return;
+
 	new_entry = brute_new_timestamp();
 	if (WARN(!new_entry, "Cannot allocate last crash timestamp\n"))
 		return;
@@ -461,8 +585,10 @@ static void brute_task_fatal_signal(const kernel_siginfo_t *siginfo)
 		crash_period = brute_get_crash_period(new_entry, old_entry);
 		kfree(old_entry);

-		if (crash_period < (u64)brute_crash_period_threshold)
+		if (crash_period < (u64)brute_crash_period_threshold) {
 			pr_warn("Fork brute force attack detected\n");
+			brute_kill_offending_tasks(*stats);
+		}
 	}

 	spin_unlock_irqrestore(&(*stats)->lock, flags);
--
2.25.1